A thiocoumarin-based colorimetric and ratiometric fluorescent probe for Hg2+ in aqueous solution and its application in live-cell imaging

Article abstract of DOI:10.1039/c8nj01491d

A new intramolecular charge transfer (ICT) based colorimetric and ratiometric fluorescent chemodosimeter for the detection of Hg²⁺ has been rationally designed and developed. The probe operates via the specific mercury-promoted desulfurization reaction of thiocoumarin to coumarin and exhibits high selectivity and sensitivity in almost pure aqueous solution (containing only 1% DMSO) with a low detection limit of 1.85 ppb. Furthermore, the probe was successfully used for the fluorescence imaging of Hg²⁺ in live cells.

Full text of DOI:10.1039/c8nj01491d

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Jason B. Benedict et al.
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A Thiocoumarin-Based Colorimetric and Ratiometric Fluorescent
Probe for Hg2+ in Aqueous Solution and Its Application in Live-Cell
Imaging
Received 00th January 20xx,
Accepted 00th January 20xx
Siyao Qin, Bo Chen, Jing Huang, and Yifeng Han*
DOI: 10.1039/x0xx00000x
A new intramolecular charge transfer (ICT) based colorimetric and ratiometric fluorescent chemodosimeter for the
detection of Hg2+ has been rationally designed and developed. The probe operates via the specific mercury-promoted
desulfurization reaction of thiocoumarin to coumarin and exhibits high selectivity and sensitivity in almost pure aqueous
solution (containing only 1% DMSO) with a low detection limit of 1.85 ppb. Furthermore, the probe was successfully used
for fluorescence imaging of Hg2+ in live cells.
www.rsc.org/
made to develop fluorescent probe for Hg2+ based on the
coordination of Hg2+ to heteroatom-based ligands,9 Hg2+
catalyzed devinylation and oxymercuration-elimination
Introduction
Mercury, one of the most ubiquitous and poisonous heavy
metals, has caused serious damage to the environment and
human health.1 Mercury ions are not biodegradable, and
hence could be concentrated through the food chain in the
tissues of fish and marine mammals.2 Excess mercury
accumulation may induce heavy damage to the central
nervous system, leading to various cognitive and motor
disorders because of the dysfunction of proteins and enzymes
caused by the strong affinity between mercury ions and sulfur-
containing organic ligands.3 Therefore, the determination of
mercury in biological and environmental samples is crucial
both to the monitoring of environmental pollution and to the
diagnosis of clinical disorders.
reactions,10 and Hg2+ catalyzed desulfurization reactions,
including hydrolysis, cyclizations, and eliminations.11 A portion
of them are demonstrated to be potentially useful for
bioanalytical research. Those probes are effective for cellular
imaging but most of them are intensity-based and show ‘‘turn-
on’’ or “turn-off” fluorescence changes. As the fluorescence
intensity is significantly influenced by the excitation/emission
efficiency, the probe concentration, the sample environments
such as temperature, pH, and medium polarity, as well as the
instrumental set-ups, such intensity-based fluorescent probes
are not suitable for use in quantitative detection. In contrast,
ratiometric fluorescent measurement, which uses the ratio of
two fluorescent bands instead of the absolute intensity of one
band for quantification, provides the practical advantage of
built-in variability correction.12 Therefore, the development of
highly sensitive and selective ratiometric fluorescent probe for
the accurate detection of Hg2+ is still in great demand.
A few ratiometric Hg2+ probes have been already reported.13
However, most of them are Förster resonance energy transfer
(FRET)-based scaffolds by combination of two different
chromophores which have low aqueous solubility with need
for laborious probe synthesis processes. The probes enabling
ratiometric imaging of Hg2+ in live-cell by one fluorophore are
still demanding.
Traditional analytical techniques, such as atomic absorption-
emission spectrometry,4 inductively coupled plasma mass
spectrometry,5 reversed-phase high-performance liquid
chromatography,6 and anodic stripping voltammetry,7 used for
quantification of mercury species, often require expensive and
sophisticated instrumentation, or sample preparation, and are
therefore not suitable for real-time and in situ analysis. While
in contrast, small fluorescent probe methods are getting more
and more demanding in the bioanalytical field because of their
good cell membrane permeability, ease of application in
solution, as well as their high sensitivity to and selectivity for
trace analytes with spatial and temporal resolution.8
Coumarin dye is a well-known chromophore exhibiting
strong intramolecular charge transfer (ICT) due to “push-pull”
substituent pairs which results in a large spectral shift.14 To
date, a number of coumarin-based fluorescent probes have
been developed for different analytes through the modulation
of ICT efficiency. Accordingly, many coumarin-based
fluorescent probes for Hg2+ have also been designed and
developed.
Over the past several years, considerable efforts have been
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coumarin (4.007 eV), which means that the thiocoumarin and
coumarin absorb difference wavelength of light and the
appearance of two prominent emission peaks could also be
expected of them.
Our research group is actively engaged in the development
of novel fluorescent probes for heavy metal ions.15 Herein, we
disclose such a simple probe (thiocoumarin, MS4) that enables
detection of Hg2+ in aqueous solution as well as in live cells
through colorimetric and ratiometric response.
Experimental section
General methods
All the solvents were of analytic grade. NMR experiments were
carried out on a Bruker AV-400 NMR spectrometer with
chemical shifts reported in ppm (in CDCl3, d6-DMSO, or TMS as
an internal standard). Mass spectra were measured on an
Agilent 1290 LC-MS spectrometer. All pH measurements were
made with a Sartorius basic pH-Meter PB-10. Fluorescence
spectra were determined on a PerkinElmer LS55 Fluorescence
spectrophotometer. Absorption spectra were collected on a
Shimadzu UV 2501(PC)S UV-Visible spectrophotometer. All the
cation solutions were prepared from NaCl, KCl, CaCl2, MgCl2,
ZnCl2, CoCl2, CrCl2, CuCl2, FeCl2, FeCl3, NiCl2, Pb(OAc)2, SnCl4,
AgNO3, and Hg(ClO4)2 in distilled water, with a concentration
of 1 mM, respectively. The excitation and emission widths for
MS4 were all 3/3.
Fig. 1 Desulfurization reaction-based fluorescent Hg2+ probes (a) “turn-on”
sensing (previously reported) and (b) “ratiometric” sensing (this work).
The first thiocoumarin derivative fluorescent probe for Hg2+
was reported by Chang and co-works based on the
desulfurization of thiocoumarin derivative to coumarin
fluorophore.11i Subsequently, several thiocoumarin derivative
based Hg2+ probes appeared, but all of them are intensity-
based and show “turn-on” fluorescence changes (Fig. 1).11j, 11k
To develop novel highly sensitive and fast responsive
fluorescent probe with efficient colorimetric and ratiometric
response for Hg2+, we report probe MS4 (mercury sensor)
which was developed by sulfur substituted coumarin in which
7-amine donor and 2-carbonyl acceptor make a typical ICT
system. We envisioned that modifying the 2-carbonyl acceptor
into a more electron-withdrawing thiocarbonyl group that
could be specifically desulfurization by Hg2+ back to the
carbonyl group would affect its ICT effect, and thereby
resulted in colorimetric and ratiometric response in both color
and fluorescence. Further Density Functional Theory (DFT)-
based theoretical calculations by Gaussian 09 program
package also support this ratiometric fluorescence changes
between thiocoumarin and coumarin (Fig. 2). As shown in Fig.
2, the calculations revealed that the HOMO-LUMO energy gap
of the thiocoumarin was lower (3.448 eV) compared to the
Synthesis
7-(diethylamino)-2H-chromen-2-one (2)
: was synthesized
according to the literature.16 1H NMR (400 MHz, Chloroform-d)
δ 7.52 (d, J = 9.2 Hz, 1H), 7.23 (d, J = 8.8 Hz, 1H), 6.55 (d, J = 8.8
Hz, 1H), 6.48 (s, 1H), 6.02 (d, J = 9.2 Hz, 1H), 3.40 (q, J = 7.1 Hz,
4H), 1.20 (t, J = 7.1 Hz, 6H).
7-(diethylamino)-2H-chromene-2-thione (3, MS4)
:
The
mixture of compound 2 (500 mg, 2.30 mmol) and Lawesson's
reagent (930 mg, 2.30 mmol) in dry PhMe (10 mL) was
refluxed under nitrogen atmosphere for 6 h until all starting
material got consumed which was monitored by TLC analysis.
The solvent was removed under vacuum and the product was
purified by flash chromatography using petroleum ether/DCM
1
(480 mg, 89%) as a red solid. H
(2:1, v/v) as eluant to give
3
NMR (400 MHz, Chloroform-d) δ 7.28 (d, J = 9.2 Hz, 2H), 6.92
(d, J = 8.9 Hz, 1H), 6.63 (d, J = 5.6 Hz, 1H), 6.62 (s, 1H), 3.40 (q, J
= 6.8 Hz, 4H), 1.20 (t, J = 5.6 Hz, 6H). 13C NMR (100 MHz,
Chloroform-d) δ 197.63, 159.45, 151.13, 136.00, 128.84,
123.11, 110.63, 110.33, 97.09, 44.94, 12.33. MS (ESI): Calcd.
for ([M+H])+, 234.3; Found, 234.3. HR-MS (ESI): Calcd. for
([M+H])+, 234.0953; Found, 234.0959.
Cell culture and imaging
HeLa cells were cultured in DMEM (Invitrogen, Carlsbad, CA),
supplemented with 10% fetal bovine serum in a humidified
atmosphere of 5% CO2 at 37 oC. For imaging experiments,
Fig. 2 Energy gaps (∆E L/H), HOMO and LUMO energy levels of the frontier
molecular orbitals of thiocoumarin and coumarin.
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e
xponentially growing cells were seeded in 24-well plate. Cells
were cultured at 37 oC in a 5% CO2 atmosphere for 24 h before
they were exposed to reagents.
For labeling, the medium was removed and cells were rinsed
three times with PBS. Then HeLa cells were incubated with
o
MS4 (10 μM) in PBS (containing 1% EtOH) at 37 C for 30 min
as control. For Hg2+ imaging, another set of HeLa cells was
preloaded with MS4 (10 μM) in PBS (containing 1% EtOH) at 37
oC for 30 min, rinsed three times with PBS and further treated
o
with 20 μM of Hg2+ in PBS at 37 C for additional 30 min. Cells
were rinsed three times with PBS and bathed in it, then
imaging was carried out.
Results and discussion
Fig. 4 Fluorescence spectra of MS4 (5.0 μM) in PBS buffer solution (10 mM,
pH 7.4, containing 1% DMSO) in the presence of different concentrations of
Hg2+ (0-10.0 equiv.). (λex = 420 nm). Inset 1: the ratio of the fluorescent
intensity of MS4 at 477 nm and 543 nm (I477 nm/I543 nm) as a function of
Hg2+ concentration (0-10.0 equiv.). Inset 2: cuvette images of probe MS4
Scheme 1 Synthesis of MS4: (a) i) diethyl malonate, piperidine, EtOH, reflux,
6 h; ii) conc. HCl, AcOH, reflux, 12 h, 89% in two steps; (b) Lawesson's
Reagent, PhMe, reflux, 6 h, 89%.
before and after addition of Hg2+ taken under a hand-held UV-lamp (λex
=
365 nm).
As shown in Scheme 1, MS4 can be readily prepared in two
dramatically decreased, and a new band at 386 nm, which is
characteristic for the coumarin fluorophore (Fig. 3) (blue-shift
about 99 nm with a marked color change from light yellow to
steps under facile conditions with good yields starting with
commercially
available
4-(diethylamino)-2-hydroxybenz-
aldehyde. The product (MS4) was well characterized by 1H, 13
C
colorless), indicating that compound
2 (coumarin) was formed
NMR, MS, and X-ray (ESI†).
in the present of Hg2+. Moreover, the UV-vis titration curve
revealed that the ratio of absorbance of MS4 at 386 nm and
485 nm (A386 nm/A485 nm) increased linearly with increasing
concentrations of Hg2+ (Fig. 3, and S2, ESI†) and further
smoothly increased until a maximum was reached up to 2.0
UV-vis and fluorescence spectral responses of MS4 to Hg2+
With MS4 in hand, we firstly assessed the UV-vis spectroscopic
properties of MS4 in PBS buffer solution (10 mM, pH 7.4,
containing 1% DMSO) (Fig. 3 and S1, ESI†). MS4 (10.0 μM)
displayed a moderate UV-vis absorption around 485 nm. Upon
incremental addition of Hg2+ (0-3.0 equiv.), the peak at 485 nm
equiv. of Hg2+. Furthermore, a good linear relationship (R2
=
0.99) was observed between the changes in the absorbance at
386 nm (A386 nm) with Hg2+ in the range of 0-15.0 μM (Fig. 3
and S3, ESI†).
The emission spectra of MS4 and its fluorescent titration
with Hg2+ were also recorded in PBS buffer solution (10 mM,
pH 7.4, containing 1% DMSO) (Fig. 4 and S4, ESI†). As
expected, MS4 alone is highly yellow-fluorescent with an
emission band centered at 543 nm (λex = 420 nm, λem = 543
nm, Φ = 7.9%, Table S1, ESI†) due to the effecꢀve “push-pull”
effect from 7-diethylamine donor to 2-thiocarbonyl acceptor.
However, upon progressive addition of Hg2+ (0-10.0 equiv.),
the emission band at 543 nm rapidly decreased and a new
emission band centered at 477 nm appeared instantly (blue
shift about 66 nm with an obvious emission color change from
yellow to blue), which was deduced to be attributed to the
formation of the coumarin fluorophore by Hg2+-mediated
desulfurization of the thiocarbonyl group to a less electron-
withdrawing carbonyl group (Fig. 1 and 4). Moreover, the
fluorescence titration curve revealed that the ratio of the
fluorescence intensity of MS4 at 477 nm and 543 nm (I477
Fig. 3 Absorption spectra of MS4 (10.0 μM) in PBS buffer solution (10 mM,
pH 7.4, containing 1% DMSO) in the presence of different concentrations of nm/I543 nm) increased linearly with increasing concentrations
Hg2+ (0-3.0 equiv.). Inset 1: the UV-vis absorption of MS4 (10.0 μM) at 485
of Hg2+ (Fig. 4, and S4, ESI†) and further smoothly increased
until a maximum was reached up to 2.0 equiv. of Hg2+ (λex
nm and 386 nm as a function of Hg2+ concentration. Inset 2: cuvette images
=
of probe MS4 before and after addition of Hg2+
.
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MS4 at 477 nm and 543 nm (I477 nm/I543 nm) as a function of response time
(λex = 420 nm).
420 nm, λem = 477 nm, Φ = 8.1%, Table S1, ESI†), which is
agree with the UV-vis response. For practical purposes, the
detection limit of MS4 for the analysis of Hg2+ was also an
important parameter. The fluorescence titration curve
revealed that the ratio of the fluorescence intensity of MS4 at
477 nm and 543 nm (I477 nm/I543 nm) increased linearly with
the amount of Hg2+ in the range of 0-1.5 μM (R2 = 0.99) (Fig.
S5, ESI). Thus, the detection limit of MS4 for Hg2+ was
calculated to be 9.23
×
10-9 M (Hg2+ content = 1.85 ppb), which
was below the USA Environmental Protection Agency (EPA)
threshold in safe drinking water (2.0 ppb).17 These results
showed that MS4 could be a sensitive fluorescent probe for
the quantitative detection of Hg2+.
Scheme 2 Fluorescent ratiometric sensing of Hg2+ by MS4.
nm and 543 nm (I477 nm/I543 nm) remarkably increased to their
maximum value within the 5 minutes (Fig. 5b), but the
fluorescent response mode is just not as typical ratiometric
changes in which the fluorescence intensity of two fluorescent
bands will change simultaneously.18
Time-dependent experiment of MS4
Subsequently, the time-dependence of fluorescence was also
evaluated in the presence of Hg2+ in PBS buffer solution (10
mM, pH 7.4, containing 1% DMSO) (Fig. 5). The result shows
that even the ratio of the fluorescent intensity of MS4 at 477
Actually, as shown in Fig. 5a, the fluorescence intensity of
MS4 at 543 nm (I543 nm) decreased immediately from 590 to
almost zero at first one minute after the addition of Hg2+, and
then the fluorescence intensity of MS4 at 477 nm (I477 nm)
increased slowly to maximum value. This interesting
fluorescent response indicated that the typical Hg2+-catalyzed
desulfurization reaction might be two-step with different rates
of reaction (one slow and one fast). Further, the time-
dependent UV-vis response of MS4 towards Hg2+ was also
conducted. As shown in Fig. S6, ESI†, the UV-vis absorption
peak of MS4 at 485 nm red-shifted to 515 nm after 1 min of
the addition of Hg2+, and then the peak at 515 nm decreased
as time lapsed, and a new band at 386 nm intensified
gradually. These results are in agreement with the
fluorescence response. Accordingly, the plausible mechanism
of the mercury ions induced fluorescence response is shown in
Scheme 2. In Step 1, MS4 forms relatively stable complex with
Hg2+ by strong S-Hg bond, and the resultant MS4-Hg complex
should be no fluorescence emission. Subsequently in Step 2,
the desulfurization of thiocarbonyl group by water to afford
coumarin (compound
2) occurred slowly. Based on the
fluorescence response, Step 1 should be the fast step and the
Step 2 should be the slow step of the desulfurization reaction,
which has also be confirmed by NMR titration experiments (Fig.
S7, ESI).
The selection and competition experiments of MS4
Continuing on, the fluorescence titration of MS4 with various
metal ions was conducted to examine the selectivity (Fig. 6).
To our delight, the turn-on fluorescent response of MS4 is
highly specific for Hg2+ and no obvious change of fluorescent
emission was observed when it is treated with common alkali,
alkaline-earth metal ions, and transition metal ions, such as
Na+, K+, Ca2+, Mg2+, Zn2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Ni2+, Pb2+,
and Sn4+. It should be mentioned that as most of the previous
reported Hg2+-fluorescent probe, Ag+ ions also has a slight
fluorescent response to MS4 (Fig. 6 and Fig. S8, ESI†).
However, the addition of NaCl to the tested buffer solution
(PBS buffer solution, 10 mM, pH 7.4, containing 1% DMSO) is
Fig. 5 (a) Time-dependent fluorescence spectra of MS4 (5.0 μM) in PBS
buffer solution (10 mM, pH 7.4, containing 1% DMSO) in the presence of 2.0
equiv. of Hg2+ (λex = 420 nm). (b) The ratio of the fluorescent intensity of
4 | J. Name., 2012, 00, 1-3
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the simple but useful method to distinguish Hg2+ from Ag+
because of the high binding affinity between Ag+ and Cl- (AgCl,
Ksp = 1.8
× ,
10-10
Fig. 7 The ratio of the fluorescent intensity of MS4 (5.0 μM) at 477 nm and
543 nm (I477 nm/I543 nm) alone and in the presence of Hg2+ (2.0 equiv.) as a
function of pH value (λex = 420 nm).
Fig. 6 The ratio of the fluorescent intensity of MS4 at 477 nm and 543 nm
(I477 nm/I543 nm) to various metal ions (including Na+, K+, Ca2+, Mg2+, Zn2+
,
Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Ni2+, Pb2+, Sn4+, Ag+, and Hg2+). Black bars represent
the addition of 2.0 equiv. of the appropriate metal ions to a 5.0 μM solution
of MS4 (in PBS buffer solution, 10 mM, pH 7.4, containing 1% DMSO). Red
bars represent the addition of 2.0 equiv. of Hg2+ to the solutions containing
MS4 (5.0 μM) and the appropriated metal ions (2.0 equiv.) (λex = 420 nm).
see Fig. S9, ESI†). Moreover, the following compeꢀng
experiments showed that no significant response in the
fluorescence intensity was found by comparison with that
without the other metal ions besides Hg2+, which implies that
MS4 bears high selectivity for Hg2+ in the presence of other
competitive metal ions (Fig. 6 and Fig. S10, ESI†).
Fig. 8 Fluorescence images of HeLa cells with MS4 (10.0 μM) and Hg2+ (0,
and 20.0 μM, respectively) for 30 min (λex = 405 nm).
and S11, ESI†), which was due to the fact that sulfur atom
could lost its nucleophilicity by protonation in acidic
environment. These results are also consistent with the
proposed mechanism (Scheme 2). However, satisfactory Hg2+-
sensing abilities were exhibited in the range of pH from 6.0 to
9.0 (Fig. 7 and S12, ESI†), indicaꢀng that MS4 could be used in
living cells without interference from pH effects.
pH titration of MS4
Moreover, the Hg2+-sensing ability of MS4 at a wide range of
pH values was investigated. As shown in Fig. 7, S11 and S12,
ESI†, MS4 alone was inert to pH in the range of 4.0-9.0. On the
other hand, in the presence of Hg2+, the fluorescence response
scope of MS4 decreased when the pH value decreased (Fig. 7
Cell imaging
Due to the favorable properties of MS4 in vitro, its potential
application in living cells was studied. For this purpose, HeLa
cells were first incubated with MS4 (10.0 μM) for 30 min, then
Hg2+ (0, and 20.0 μM, respectively) was added and incubated
for another 30 min at 37 oC, and the fluorescence images were
taken. As shown in Fig. 8, cells without the treatment of Hg2+
showed very dim fluorescence in the blue channel and a strong
fluorescence in the red channel. Whereas, incubation with
both MS4 (10.0 μM) and Hg2+ (20.0 μM) for 30 min led to a
distinct increase in blue channel fluorescence, accompanied by
the dramatic drop of red channel fluorescence. These obvious
changes indicated that MS4 was a cell membrane permeable
and capable of imaging Hg2+ in living cells. Furthermore, the
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(a) M. Vedamalai, S. P. Wu, Org. Biomol. Chem., 2012, 10,
series of bright field images shown that the cells were intact,
healthily spread and adherent during and after the labeling
process with MS4 indicating that the probe has no cytotoxic
effect.
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Conclusions
In conclusion, we have rationally developed a new and simple
ICT-based sensitive fluorescent probe for the selective
detection of Hg2+ via mercury-triggered desulfurization
reaction. The probe has the unique advantage of easy-
preparation, good water solubility, low detection limit, and
fast and specific response towards Hg2+ under mild conditions.
Furthermore, fluorescence imaging of Hg2+ in live cells
indicated that this probe might be favorable for biological
applications.
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This work was supported by Zhejiang Provincial Natural
Science Foundation of China (Grant No. LY17B070008), the
Graduate Innovative Research Program of Zhejiang Sci-Tech
University (YCX16003), and the Project Grants 521 Talents
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A new intramolecular charge transfer (ICT) based colorimetric and ratiometric fluorescent
chemodosimeter for the detection of Hg2+ has been rationally designed and developed.